1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with in-plane switching mode.
2. Description of the Related Art
Of the various liquid crystal display (LCD) devices, those using the twisted nematic mode (TN mode) have low operation voltage and low power consumption, and in recent years have been widely used. A TN mode cell has a liquid crystal layer interposed between first and second glass substrates on which transparent electrodes are formed. The longitudinal axis of molecules of the liquid crystal are parallel to the planes of the substrates. The first and second glass substrates are arranged for a twist angle of almost 90.degree..
A problem associated with the TN mode-LCD described above, is that the best angle of view cannot be set in the direction normal to the plane of the glass substrate. If the direction of the best angle of view from the glass substrate is defined as upward, the angle of view is symmetric in the horizontal direction whereas the angles of view are unsymmetric in the vertical direction.
To solve the problem, liquid crystal display device with In-Plane Switching (IPS) mode (refer to Principle and Characteristics of Electro-optical Behaviour with In-Plane Switching mode; Asia Display '95, pp 578-580, Hitachi, Japan) is proposed.
FIG. 1 shows a liquid crystal cell with a IPS mode when a voltage is not applied to the cell. FIG. 2 shows the liquid crystal cell shown in FIG. 1 when the voltage is applied to the cell. In FIG. 1, on the outsides of first and second substrates 22a and 22b, a polarizer 21a and a analyzer 21b are arranged, respectively. The polarization axes of the polarizer 21a and analyzer 21b are orthogonal to each other. On the first substrate 22a, a pixel electrode 23 and a counter electrode 24 are formed so that they are spaced to a selective space and are parallel to each other. On a surface of the first substrate 22a having the electrodes formed thereon is formed a homogeneous alignment film (not shown). On a surface of the second substrate 22b is also formed a homogeneous alignment film (not shown). Both alignment films are anti-parallel to each other. A liquid crystal layer 25 having liquid crystal molecules 25a is interposed between the first and second substrates 22a and 22b.
As shown in FIG. 1, when the voltage is not applied to the cell, the arrangement of the liquid crystal molecules 25a are homogeneous and their optical axes are in good agreement with the polarization axis of the polarizer 21a. Therefore, incident light becomes linearly polarized light at the polarizer 21a to reach the analyzer 21b due to the homogeneous arrangement of the liquid crystal molecules 25a, and a dark state exists because the linearly polarized light is not in good agreement with the polarization axis of the analyzer 21b through which the light must exit. On the other hand, as shown in FIG. 2, when the voltage is applied to the electrodes 23 and 24, electric field E direction of which is parallel to the plane of the first substrate 22a, is formed between the pixel and counter electrode 23 and 24. Therefore, the longitudinal axes of the liquid crystal molecules 25a are arranged so that they parallel to the electric field E. Under the influence of the electric field E, the optical axes of the liquid crystal molecules 25a deviate by an angle of about 45.degree. from the polarization axis of the polarizer 21a. As a result, the incident light which have been changed to linearly polarized light at the polarizer 21a, changes to elliptically polarized light, and a bright state exists because most of the incident light passes through the analyzer 21b.
This IPS mode-LCD above described has wide view-angle characteristic which are better than the TN mode-LCD while the former has a longer response time than the latter, since the liquid crystal molecules turn in keeping their longitudinal axis parallel to the plane of the substrate and switch light transmission. Furthermore, since the electrodes are formed only on the first substrate, the electric field E becomes fringe field as close to the second substrate. As a result, the liquid crystal molecules which close to the second substrate have a further slow response time, and their the optical axes deviate within about 45.degree. from the polarization axis of the polarizer, thereby decreasing transmittance.